The present invention generally relates to implantable medical devices and more particularly to cardiac stimulation devices, e.g., pacemakers, ICDs and the like, using fusion avoidance methods to improve the performance of automatic threshold determination techniques which are employed by such devices.
Implantable pacemakers generate electrical stimulation pulses and deliver such stimulation pulses to atrial and/or ventricular muscle tissue of a patient""s heart at a prescribed rate and/or rhythm when, through disease or other causes, the heart is not able to maintain the prescribed heart rate or rhythm on its own. When the delivered electrical stimuli are of sufficient energy, they cause the cardiac muscle tissue to depolarize, and therefore contract, thereby forcing the heart rate or rhythm to track the delivery of the electrical stimuli. When the delivered electrical stimuli are of insufficient energy, depolarization does not occur, and the heart rate or rhythm is not controlled by the pacemaker. Hence, for the pacemaker to perform its intended function, it is critically important that the delivered electrical stimuli be of sufficient energy to depolarize the cardiac tissue, a condition known as xe2x80x9ccapturexe2x80x9d.
The energy of the electrical stimuli generated by an implanted pacemaker is derived from the energy stored in the pacemaker""s battery. The pacemaker""s battery has a limited amount of energy stored therein, and the generation of electrical stimuli represents by far the greatest drain of such energy. In order to preserve this limited energy and prolong the life of the battery, it is known in the art to adjust the energy of the delivered electrical stimuli, generally the amplitude of the stimulation voltage, so that it is just sufficient to cause capture, with an appropriate safety margin. See, e.g., U.S. Pat. Nos. 3,949,758 and 4,686,988. The amount of energy needed to effectuate capture is known as the capture xe2x80x9cthresholdxe2x80x9d, and electrical stimuli of energy less than the capture threshold do not bring about capture, while electrical stimuli of energy greater than the capture threshold do bring about capture. By adjusting the energy of the electrical stimuli so that it is always greater than the capture threshold, but not too much greater, the limited energy of the pacemaker battery may thus be preserved. The battery energy is preserved because: (1) electrical stimuli of insufficient energy to cause capture (electrical stimuli below threshold), which stimuli represent wasted energy, are rarely generated; and (2) electrical stimuli of excessive energy (energy much greater than the capture threshold), which excess energy not only represents wasted energy, but also energy that may disadvantageously cause pectoral stimulation and/or sensation, are also rarely generated.
In order to determine the capture threshold, prior art devices periodically enter into an autothreshold determination mode in which a stimulation pulse is delivered and the electrical signals from the cardiac tissue are monitored to determine if depolarization has occurred. Generally, autothreshold determinations decrease the amplitude of a stimulation pulse (from an initial maximum value) until capture is lost and then use this loss of capture amplitude value plus a safety margin value as the new capture threshold value. However, this procedure assumes that the cardiac signals only represent evoked responses to the stimulation pulses, e.g., intrinsic P-waves are not present. However, in certain cases referred to as fusion, intrinsic P-waves may coincide with the stimulation pulses and thus both events contribute to the sensed electrical activity of the cardiac tissue. Accordingly in such cases, an autothreshold determination could be unreliable since the intrinsic event may mask the evoked response. U.S. Pat. No. 5,766,229 to Bornzin, whose teachings are incorporated herein by reference, teaches that fusion must be prevented in order to provide a reliable atrial autothreshold determination. In the Bornzin patent, fusion is prevented by overdriving the atrial pacing rate based upon a variance of the intrinsic rate. While this technique reduces the probability of a fusion beat, it is believed that this technique will not eliminate fusion. Accordingly, what is needed is a method for providing an additional level of certainty that fusion beats will not interfere with the autothreshold determination process.
The present invention is directed to an implantable cardiac stimulation device, e.g., a pacemaker, ICD or the like, and associated method for altering the stimulation regime during determination of a capture threshold level. In order to avoid the potential for fusion beats which would interfere with a capture threshold level determination, a two-fold approach is used. First, the atrial refractoriness of the atrium of the patient""s heart is extended by supplying a secondary atrial stimulation pulse during a determined time period following delivery of a primary atrial stimulation pulse. Second, the atrium is overdriven by providing an atrial stimulation pulse at a rate in excess of the intrinsic heart rate. Accordingly, the potential for fusion beats is significantly decreased and the reliability of the determined capture threshold level is increased. In a preferred embodiment, two closely placed stimulation pulses, i.e., a first primary atrial stimulation pulse and a secondary atrial stimulation pulse, are provided to an atrium of the patient""s heart and a second primary atrial stimulation pulse set to a test level is delivered faster than the heart""s intrinsic rate. During this pacing regime, the potential for fusion beats is decreased and accordingly, sensing the presence or absence of evoked responses can accurately determine the capture threshold level.
A preferred method for suppressing fusion beats to thereby facilitate the periodic determination of a capture threshold level for pacing the atrium of a patient""s heart comprises (1) determining the intrinsic cardiac rate, (2) delivering a first primary atrial stimulation pulse having a first amplitude level to an atrium of the patient""s heart, (3) timing a delay time period following the primary atrial stimulation pulse, (4) delivering a secondary atrial pacing pulse having a second amplitude level to the atrium of the patient""s heart at the end of the delay time period, and (4) delivering a second primary atrial stimulation pulse having a third amplitude level to the atrium of the patient""s heart, whereby the second primary atrial stimulation pulse is delayed from the first primary atrial stimulation pulse by a time period defining a cardiac rate in excess of the intrinsic cardiac rate, whereby the likelihood of fusion beats is minimized.
In a further aspect of a preferred embodiment, the first primary atrial stimulation pulse and the secondary atrial stimulation pulse are both configured to have high amplitudes, selected to ensure capture of the atrium of the patient""s heart. In an alternative embodiment, the first primary atrial pulse may be set to an amplitude corresponding to the second primary atrial stimulation pulse which is set to the test level during the capture threshold determination.
The capture threshold determination may run continuously until completed or one determination portion may be done at a time. Specifically, the two fast pulses, i.e., the first primary atrial stimulation pulse and the secondary atrial stimulation pulse, will precede each second primary atrial stimulation pulse which is used to determine capture. However, in a first embodiment, the next second primary atrial stimulation pulse (used to determine capture) will not occur until two more fast pulses are delivered. Alternatively, the second primary atrial stimulation pulse can serve multiple functions. First, it is used to determine capture. Second, it is used as the next first primary atrial stimulation pulse and is directly followed in the same cardiac cycle by a secondary atrial stimulation pulse, i.e., it also becomes one of the two fast pulses for the next second primary atrial stimulation pulse.